The present invention relates to rotary electric motors, more particularly to the precise control of brushless permanent magnet motors.
The above-identified copending patent applications describe the challenges of developing efficient electric motor drives for vehicles, as a viable alternative to combustion engines. Electronically controlled pulsed energization of windings of motors offers the prospect of more flexible management of motor characteristics. By control of pulse width, duty cycle, and switched application of a battery source to appropriate stator windings, functional versatility that is virtually indistinguishable from alternating current synchronous motor operation can be achieved. The use of permanent magnets in conjunction with such windings is advantageous in limiting current consumption.
In a vehicle drive environment it is highly desirable to attain smooth operation over a wide speed range, while maintaining a high torque output capability at minimum power consumption. Motor structural arrangements described in the copending applications contribute to these objectives. Electromagnet core segments may be configured as isolated magnetically permeable structures in an annular ring to provide increased flux concentration. Isolation of the electromagnet core segments permits individual concentration of flux in the magnetic cores, with a minimum of flux loss or deleterious transformer interference effects with other electromagnet members.
Precision controlled performance within brushless motor applications involves the fusion of nonlinear feedforward compensation coupled with current feedback elements. However, feedforward compensation expressions typically rely heavily upon various circuit parameters, such as phase resistance, phase self-inductance and the like, which are depicted illustratively in the equivalent circuit diagram for an individual motor phase in FIG. 1. Vt (t) denotes the per-phase voltage input, Rt denotes the per-phase winding resistance, and Lt represents the per-phase self-inductance. Et (t) represents the opposing back-emf voltage of the motor per phase and can be approximated by the following expression:
Et=(Keixcfx89)sin(Ntxcex8t)
where Kei denotes the per-phase back-emf voltage coefficient, xcfx89(t) represents the rotor velocity, Nt denotes the number of permanent magnet pairs, and xcex8t(t) represents the relative displacement between the ith phase winding and a rotor reference position.
Due to phenomena affected by mechanical/manufacturing tolerances and other structural characteristics, each motor phase will exhibit a range of values for each circuit element. Factors that can affect the magnitudes of the circuit parameters include: the net flux linkage of the electromagnet core; fluctuations in the inductance of the core with respect to the electrical circuit; variations in the resistance of the phase winding due to changes in manufacturing tolerances such as the cross sectional area and winding tension; variations in the permeability of the core (related to the grade and the processing and finishing history of the material); phase winding technique (uniform or scrambled wound) or the build quality of the coils on each stator core; position of the electromagnet and permanent magnet interaction (i.e., permeance of the magnetic circuit); variations in the air gap flux density, which is dependent on the permanent magnet rotor magnet sub assembly; residual magnetic flux density; biasing magnetic field due to external magnetic fields; shape of coil wire (rectangular, circular or helical); winding factor achieved in the coil; manufacturing tolerances achieved in the core geometry which could alter the cross sectional tolerance of the core; the effective length over which the coil is wound.
Typically, motor control strategies assume uniformity of parameter values over the entire motor. One median parameter value is taken to represent all corresponding circuit elements of the motor. This lumped parameter approach often leads to degradation in tracking performance due to over/under compensation of the control strategy due to parameter value mismatch within individual phase compensation routines. Such assumed parameters are prone to even greater discrepancies with stator structures configured as autonomous ferromagnetically isolated core components. Thus, the need exists for an individualized circuit parameter compensation that accounts for the parameter variations in the separate phase windings and stator phase component structures.
The present invention fulfills this need, while maintaining the benefits of the separated and ferromagnetically isolated individual stator core element configurations such as disclosed in the copending applications. The ability of the present invention to implement a control strategy that compensates for individual phase circuit elements offers a higher degree of precision controllability since each phase control loop is closely matched with its corresponding winding and structure. This ability is obtained, at least in part, by establishing in a control system for a multiphase motor one or more sets of parameters, the parameters of each set specifically matched with characteristics of a respective stator phase. Successive switched energization of each phase winding is governed by a controller that generates signals in accordance with the parameters associated with the stator phase component for the phase winding energized. Such control provides advantages with motors of a variety of construction and can be applied to a motor in which each stator phase component comprises a ferromagnetically isolated stator electromagnet, the electromagnet core elements being separated from direct contact with each other and formed with separate phase windings.
A digital signal processor may be utilized that applies an algorithm incorporating the parameters as constant values, the parameters for a particular phase being accessed for generating the appropriate control signals for energizing that phase. Other parameters are variable in dependence upon selected states of the system, such as position, temperature and other external conditions. Alternatively, the controller may be provided with a separate loop for each phase, each loop executing a control algorithm containing the parameters for the respective phase. The algorithms may contain components based on the current sensed in each phase, the sensed position and speed of the rotor, the sensed conditions received as input signals to the controller.
The present invention is particularly advantageous in applications in which the motor is intended to track a variable user initiated input, such as electric vehicle operation. In response to torque command input signals, per-phase desired current trajectories are selected by the controller in accordance with an expression that includes the particular parameters for each phase.
Additional advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein only the preferred embodiment of the invention is shown and described, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive.